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February 2, 2011

‘Invisibility Cloak’ Uses Natural Crystals

by Sam Savage

Researchers in Britain and Denmark have unveiled an "Ëinvisibility cloak' that uses a common crystalline material known as calcite, which works by sending its two "polarizations" of light in different directions.

Cloaking involves guiding light waves in such a way that waves from a hidden object do not reach the eye. By using calcite, the researchers created an "Ëinvisibility cloak' that hid a small (inch-size), three-dimensional object. However, the size of the object being cloaked is limited only by the size of the calcite crystal, the researchers said.

The latest approach offers an easier, more scalable way to achieve invisibility than previous methods using metamaterials -- synthetic composites with optical properties not found in nature. Furthermore, a natural crystal such as calcite is able to hide objects much larger than those cloaked using metamaterials, which limits the size of the cloaking region.

Calcite is a transparent mineral with birefringent or double-refraction properties, meaning that light enters the calcite and splits into two rays of different polarizations traveling at different speeds and in different directions.

The current demonstration was performed both in the air and in a container of liquid by using two triangular pieces of calcite glued together and placed on a mirror.

"This is a huge step forward as, for the first time, the cloaking area is rendered at a size that is big enough for the observer to "Ësee' the invisible object with the naked eye," said Dr. Shuang Zhang, lead investigator from the University of Birmingham's School of Physics and Astronomy.

"By using natural crystals for the first time, rather than artificial meta-materials, we have been able to scale up the size of the cloak and can hide larger objects, thousands of times bigger than the wavelength of the light."

"Previous cloaks have succeeded at the micron level (much smaller than the thickness of a human hair) using a nano- or micro-fabricated artificial composite material. It is a very slow process to make these structures and they also restrict the size of the cloaking area. We believe that by using calcite, we can start to develop a cloak of significant size that will open avenues for future applications of cloaking devices."

The research was published February 1 in the journal Nature Communications in a paper entitled "Macroscopic Invisibility Cloak of Visible Light".